Natural gas, comprising methane and higher hydrocarbons such as ethane, propane and butane, is often recovered directly, or as an associated gas with oil production, offshore using fixed or floating platforms coupled to well heads on the seabed. The recovered natural gas is, where generally possible, fed via pipeline to on-shore gas processing facilities where such steps as purification may be carried out. For a significant proportion of the recovered natural gas, pipelining to on-shore facilities is not possible. In such cases it has become desirable to recover and liquefy the natural gas for sea transportation to on-shore facilities. Similarly, on-shore stranded natural gas is increasingly liquefied for transportation to overseas markets. The liquefaction processes typically include steps of cooling the natural gas to a very low temperature, which allows separation of at least some of the ethane, propane, butanes and other higher hydrocarbons from the methane. The liquefied products have different commercial values, but where further processing such as cracking is not feasible, typically the ethane has the lowest value, and so part is often used for power generation in the liquefaction facility and the excess is flared. Alternatively, in off shore production, the excess ethane may be recovered and transported alongside the LNG to the onshore facility. However the economics of transporting ethane in place of the more valuable liquids is less attractive. With ever-increasing pressure on maximising the utilisation of the recovered hydrocarbons coupled with a need to reduce flaring, which has become environmentally unsound, there is a need for an improved process for treating natural gas containing higher hydrocarbons.
Furthermore, so-called “rich” natural gas streams containing relatively large amounts of higher hydrocarbons, whether recovered directly or by vapourisation of rich-LNG's, often pose a problem for the processor required to meet pipeline specifications on their content and calorific value.
GB2432369 discloses a method of treating natural gas containing ethane based on the CRG process, developed originally by British Gas and licensed by Davy Process Technology Ltd. The method utilises a combination of adiabatic steam reforming over a nickel catalyst at temperatures in excess of 350° C., methanation and CO2 removal on ethane extracted from the natural gas to generate methane, which is mixed with the natural gas, which may be liquefied.
This process has a number of drawbacks including the need to separate ethane and higher hydrocarbons as well as the methanation step, which is required to convert the carbon monoxide and hydrogen formed over the nickel catalyst back into methane.
Other processes are known for adjusting the calorific value of rich natural gases but these require the use of ballasting agents such as nitrogen, which has to be separately generated and stored.